The impact of Y substitution on the 110 K high Tc phase in a Bi (Pb):2223 superconductor

ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 70 (2009) 483–488

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The impact of Y substitution on the 110 K high Tc phase in a Bi (Pb):2223 superconductor A. Sedky  Physics Department, Faculty of Science, King Faisal University, Al-Hassa 31982, P.O. Box 400, Saudi Arabia

a r t i c l e in fo

abstract

Article history: Received 16 July 2008 Received in revised form 24 November 2008 Accepted 3 December 2008

The structural and superconducting properties of Bi1.7Pb0.3Sr2Ca2xYxCu3Oy superconducting samples are investigated by X-ray diffraction (XRD), resistivity and thermoelectric power (TEP) measurements. XRD results reveal that the volume percentage of the 2223 high Tc phase decreases with an increase in Y content. The replacement of the Ca2+ ion by the Y3+ ion does not influence the tetragonal structure of the pure Bi (Pb): 2223 system and the lattice parameters vary with Y content. The results of resistivity indicate that the critical temperatures Tc of the samples decrease monotonically with an increase in Y content. Further, the critical concentration of Y to completely suppress superconductivity in the Y-doped Bi (Pb):2223 system is higher (0.60) than that reported (0.20) for the other rare-earth elements. On the other hand, the values of TEP at room temperature are found to be negative for Y ¼ 0.00 and 0.10 samples, and it changed to positive with further increase in Y content. The hole-carrier concentration per Cu ion (P) is deduced by using two different ways: the first in terms of Tc values in the superconducting state and the other in terms of TEP values in the normal state. Interestingly, it is found that the values of P deduced from the formal way are not consistent with the reported parabolic behavior for superconducting systems in the under-doped region, and consequently disagree with the general roles of substitution. However, the vice versa is recorded for the values of P deduced from the latter way. The results are discussed in terms of the possible reasons for the suppression of superconductivity in the considered system. & 2008 Elsevier Ltd. All rights reserved.

Keywords: A. Ceramics B. Chemical synthesis C. X-ray diffraction D. Electrical properties D. Transport properties

1. Introduction Since the discovery of high Tc superconductors [1], several isovalent and aliovalent substitutions for cations have been made in order to investigate the structural and superconducting properties of these materials. Interestingly, the new superconducting phases of higher Tc might result through different crystal structures incorporating substitutions of these cation sites. These results have consistently suggested that the conduction mechanism in high Tc superconductors is related to the Cu–O2 planes with a perovskite structure. Consequently, some of the studies have been directed toward the electronic structure of the Cu–O2 planes [2–4]. It is well known that there is a strong relationship between carrier concentration and critical temperature Tc in these types of materials [5–7]. Tc increases with an increase in carrier concentration until it passes through a maximum, after which it decreases, and becomes zero above a critical concentration [8]. It has been reported that most of copper oxide superconductors

 Corresponding author. Permanent address: Physics Department, Faculty of Science, Assiut University, Assiut, Egypt. E-mail address: [email protected]

0022-3697/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2008.12.006

behave like insulators in the low carrier concentration region and as superconductors in the intermediate range; however, in the heavily doped regime they are normal metal-like [9–13]. Different substitutions or changes in the oxygen stoichiometry are well known to alter the concentration of carriers, leading to dramatic changes in the normal state transport behavior, and simultaneously influence the Tc values [14]. The rare-earth elements have an incomplete inner shell, different magnetic moments and different ionic radii. Therefore, they have great importance as a substitute for studying the physical properties of high Tc superconductors. The replacement of Ca2+ in the Bi2xPbxSr2Ca2Cu3Oy superconducting system by rare-earth elements R3+ has a significant effect on their normal, mechanical and superconducting properties. Interestingly, the values of Tc are gradually decreased with the increase in R3+ content [15–18]. It is also found that Y substitution improves the connection between superconducting grains of the Y-doped Bi:2212 system, and consequently the mechanical and superconducting properties are improved [19]. However, the opposite behavior is reported for Cd substituted at Ca sites in the Bi:2223 system [20,21]. To further investigate such a possibility, we have attempted here to study the effect of Y substitution (at Ca sites) on the

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structural and transport properties of Bi1.7Pb0.3Sr2Ca2xYxCu3Oy superconductors.

Bi1.7Pb0.3Sr2Ca2-xYxCu3Oy

x = 0.50

2. Experimental details

x = 0.30

x = 0.20

Intensity (a.u)

Bi1.7Pb0.3Sr2Ca2xYxCu3Oy samples with various x values are prepared by using the solid-state reaction method. The ingredients of Bi2O3, PbO, SrO, Y2O3, CaCO3 and CuO of 4N purity are thoroughly mixed in required proportions and calcined at 820 1C in air for 24 h. This exercise is repeated three times with intermediate grinding at each stage. The resulting powder is reground, mixed, pressed into pellets and sintered at 845 1C for 150 h in air. Finally the pellets are slowly cooled to room temperature. The phase purity of the samples is examined using an X-ray diffractometer with Cu-Ka radiation. The electrical resistivity of the samples is measured by the standard four-probe technique in a closed cycle refrigerator (cryomech compressor package with cryostat Model 810-1812212, USA) within the range 10–300 K. High-quality silver paint is used for electrical contacts of the leads with the samples. Nanovoltameter Keithley 2182, current source Keithley 6220 and temperature controller 9700 (0.001 K resolution) are used in this experiment. Finally, the differential TEP (Seebeck coefficient) of the samples is measured at room temperature using the standard differential technique. A sample is sandwiched between the two copper blocks. Thermoelectric voltage is measured relative to the copper leads in the presence of a temperature gradient (DT) about 2–3 K across the measured sample.

x = 0.10

3. Results and discussion

(200)H

30

(359)L

(0020)L (357)L

(220)H

(1118)H

(208)H

(2012)H

(0014)H

(119)H (200)L

(115)H (115)L

(008)H

10

x = 0.00

50

70

Angels (2θ) Fig. 1. X-ray diffraction patterns for Bi (Pb):2223 samples.

other phases. Fig. 2(c) shows the volume fraction of the 2223 phase as a function of Y content for the Bi (Pb):2223 phase. The linear decrease of volume fraction of the 2223 phase with increasing Y content could be obtained.

3.2. Electrical resistivity

V 2223 ¼ fSI 2223 ðpeaksÞg=fSI 2223 ðpeaksÞ þ SI 2212 ðpeaksÞ þ SI other ðpeaksÞg,

(002)H (002)L

X-ray diffraction patterns (XRD) at room temperature for pure and Y-doped Bi1.7Pb0.3Sr2Ca2xYxCu3Oy samples are shown in Fig. 1. It is observed that most high-intensity peaks belong to the 2223 tetragonal phase with a few low-intensity peaks belong to the 2212 phase. The 2223 peaks are indicated by H (h k l) and those of 2212 are indicated by L (h k l). Fig. 2(a, b) shows the variation of the lattice parameters as a function of Y content. It is clear that the c-parameter decreases with an increase in Y content, whereas the a-parameter increases. Similar behavior has been observed in the reported data involving substitution of rare-earth elements at the Ca site in the Bi (Pb):2223 system [15–18]. The decrease in c-parameter with Y content is due to the smaller ionic size of Y3+ (1.02 A˚) ions compared to the Ca2+ (1.12 A˚) at the same 8-fold coordination [22]. However, it is very difficult to understand the increase in a-parameter as a result of substitution, since the length of the a-lattice parameter is controlled by the length of the in-plane Cu–O bond. The increase in the a-parameter may result from a decrease in the hole-carrier concentration per Cu ion, which weakens the Cu–O bond. Similar behavior has been reported for La:214 systems when La3+ ions are replaced by slightly larger Sr2+ ions. Anyhow the carrier concentration for the considered samples will be calculated and discussed in the next section. The volume fraction of the 2223 phase (V2223) in the Bi (Pb):2223 samples is determined using the following relation [23,24]:

(0012)H

3.1. X-ray diffraction

(1)

where I2223 is the peak intensity of the 2223 phase, I2212 is the peak intensity of 2212 phase and Iother is the peak intensity of any

Fig. 3(a) depicts electrical resistivity as a function of temperature for Bi1.7Pb0.3Sr2Ca2xYxCu3Oy samples. From this figure it can be observed that the resistivity decreases almost linearly from

ARTICLE IN PRESS A. Sedky / Journal of Physics and Chemistry of Solids 70 (2009) 483–488

5.43

37 Bi1.7Pb0.3Sr2Ca2-xYxCu3Oy

Bi1.7Pb0.3Sr2Ca2-xYxCu3Oy

36.8

5.42

C - Parameter (Å)

a-Parameter (Å)

485

5.41

5.4

36.6 36.4 36.2 36 35.8

5.39 0

0.1

0.2

0.3

0.4

0.5

0

0.6

0.1

0.2

0.3

0.4

0.5

0.6

Y Content

Y Content

Volume Percentage (2223)

90 80 70 60 50 Bi1.7Pb0.3Sr2Ca2-xYxCu3Oy

40 0

0.1

0.2

0.3 Y Content

0.4

0.5

0.6

Fig. 2. (a) Variation of a-lattice parameter with Y content for Bi (Pb):2223 samples, (b) variation of c-lattice parameters with Y content for Bi (Pb):2223 samples and (c) variation of 2223 volume fractions with Y content for Bi (Pb):2223 samples.

room temperature, beyond which it turns to superconducting state at critical temperature Tc. It is also noted that all samples exhibit a clear metallicity, which starts to slightly decrease with further Y content addition. The linear part of the r(T) curves has a positive slope dr/dT and its extrapolation to T ¼ 0 K provides the residual resistivity, say ro. While the ro is related to impurity scattering, the slope dr/dT determined from the linear region of the r(T) curves is related to carrier–carrier scattering [25]. The resistivity drop occurred in a single step, implying that the samples are mainly of single phase. But the substitution has broadened the width of the transition temperature, DTc, implying the increase in the weak links [26]. However, the values of normal state resistivity at 300 K r300, residual resistivity at 0 K ro and width of transition DTc for all samples are presented in Table 1. The values of ro, r300 and DTc generally increase with an increase in Y content. However, Tc is observed at 108 K for the pure sample and it decreased to 24 K for the x ¼ 0.50 sample, see Fig. 3(b). We would like to mention here about the substitution of R3+ in the Bi (Pb):2223 system as reported in Refs. [15–18]. The reported data of reduced critical temperature [Tc(x)/Tc(0)] against rareearth elements are drawn and shown in Fig. 3(c). For comparison, our results are enclosed in the same figure. It is clear that the critical concentration of Y to completely suppress superconductivity in Y-doped Bi (Pb):2223 samples is higher (0.60) than that reported (0.20) for Sm and Nd elements. This behavior indicates that Y, compared with Sm, Nd and Pr, has a higher solubility in the Bi (Pb):2223 system and is less detrimental to superconductivity. Different solubilities of rare-earth elements R3+ in the Bi (Pb):2223 system can be understood by a comparison of their ionic radii of 1.02, 1.09, 1.12 and 1.18 A˚ at 8-fold coordination for Y, Sm, Nd and La with respect to 1.12 A˚ of Ca2+ [22]. The solubility

of R3+ in the Bi (Pb):2223 system decreases with an increase in ionic radius, as one moves towards the right in lanthanide series. Fig. 3(d) shows the values of [Tc(x)/Tc(0)] as a function of Y concentration. For comparison, in the same figure, we have also included similar results for Bi:2212 with Y substitution as reported in Ref. [13]. The decrease in [Tc(x)/Tc(0)] of the Y-doped Bi (Pb):2223 samples with an increase in Y apparently follow a similar behavior to that of the Y-doped Bi:2212 system. 3.3. Thermoelectric power and hole carrier Earlier Presland et al. [27] have found that the phase diagrams for pure and doped La:214 cuprate systems are well described by the following parabolic formula: ½T c ðxÞ=T c ð0Þ ¼ 1  82:6ðP  0:16Þ2 ,

(2)

where Tc(0) and Tc(x) are the critical temperature in the superconducting state for pure and doped samples, respectively. P is the hole-carrier concentration per Cu ion. Therefore, P can be established from the values of Tc(0) and Tc(x) using the above parabolic relationship. Later, Obertelli et al. [28] have found that the above formula is suitable and generally can be applied for the Bi and TI superconducting systems [28–30]. Fig. 4(a) shows the variation of hole-carrier concentration per Cu ion as a function of Y content. It is clear that P increases and almost linearly up to x ¼ 0.50, which is in disagreement with our previous explanation for the general roles of substitution. However, although we cannot understand this unusual behavior under the present situation, we can mention here that the above relation is applied in terms of the values of Tc(0) and Tc(x) in the superconducting state and it is

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120

40 Bi1.7Pb0.3Sr2Ca2-xYxCu3Oy

x = 0.50

Critical Temp. (K)

Resistivity (mohm.cm)

100 30 x = 0.40

20 x = 0.30

10

60 40

x = 0.20

20

x = 0.10

Bi1.7Pb0.3Sr2Ca2-xYxCu3Oy

x = 0.00

0

0 0

150 100 200 Temperature (K)

50

250

300

0

1.2

0.1

0.2

0.3 Y Content

0.4

0.5

0.6

1.2 Bi1.7Pb0.3Sr2Ca2-xRxCu3Oy

Ca/Y

Reduced Critical Temperature

Reduced Critical Temperature

80

1 Ca/Pr Ca/Sm

0.8 Ca/Nd

0.6

0.4

0.2

Bi: 2212 (Ca/Y)

0.8

0.6

0.4

0.2 0

0.1

0.2

0.3

0.4

0.5

0.6

Bi: 2223 (Ca/Y)

1

0

0.1

0.2

0.3

0.4

0.5

0.6

Y Content

Rare-earth content

Fig. 3. (a) Resistivity versus temperature for Bi (Pb):2223 samples. (b) Tc versus Y content for Bi (Pb):2223 samples. (c) [Tc(x)/Tc(0)] versus rare-earth elements R content of Bi (Pb):2223 samples (Ca/Y, present work & Ca/Pr, Rajvir Singth et al., Supercond. Sci. Technol. (1997) & Ca/Sm, Nada Kishore et al., Physica C 1995 & Ca/Nd, A.I. Malik et al., Physica C 2002). (d) Reduced temperature [Tc(x)/Tc(0)] as a function of Y content in the Bi (Pb):2223 and Bi:2212 systems (Bi:2223, present work & Bi:2212, P. Mandal et al., Physical Review B 1991).

Table 1 Values of normal resistivity, residual resistivity and width of transition for pure and Y-doped Bi (Pb):2223 samples. Y content (x)

r300 (mO cm)

ro (mO cm)

DTc (K)

0.00 0.10 0.20 0.30 0.40 0.50

4.65 5.8 8.4 12.8 23.6 37

0.51 1.41 1.60 4.00 18 22

12 11 8 11 31 41

generic to the whole class of high Tc superconductors [27,28], though accurate values of P are difficult to extract near the peak in this parabola. Actually, the room-temperature thermoelectric power (TEP) does not have this problem, and it has been found to display a dependence on hole-carrier concentration common to high Tc superconductors. Therefore, we have measured the TEP at room temperature and the results are shown in Fig. 4(b). It is clear that the values of TEP are negative for Y ¼ 0.00 and 0.1 samples, and it changed to positive with increase in Y up to 0.50. The

change of TEP from negative to positive with further increase in Y content can be explained by assuming the sum of the contributions from the majority hole carriers, which are thought to be responsible for superconductivity in these materials as well as electrons. In pure and low Y-doped samples, the electron contribution to TEP dominates because of strong dependence of electron mobility [17,31]. This might be the reason for the negative values of TEP at room temperature in these samples. This has been interpreted in terms of strong Coulomb interaction and correlation effects between the carriers within the two-band Hubbard model [17,32,33]. To complete this scenario, the hole concentration per Cu ions (P) is also calculated from the following TEP expression. According to the two-band Hubbard model, the hole concentration per Cu ion (P) can be expressed in terms of the TEP at room temperature as follows [17,29,33,34]:       mV KB 1P S ¼  ln 2 . (3) ln K 2P e The hole-carrier concentration per Cu ion, calculated using the above formula, is plotted as a function of Y content and shown in Fig. 4(a). From this figure we can notice that P decreases and

ARTICLE IN PRESS A. Sedky / Journal of Physics and Chemistry of Solids 70 (2009) 483–488

Let us now return to the mechanism of excess oxygen in the BSCCO system. In the considered system, the electrons are transferred from the Cu to the BiO layer, leading to the formation of holes on Cu layers and electrons on Bi layers according to the following reaction [38,39]:

0.3

Hole Concentration

0.25 0.2

Bi3þ þCu2þ ! Bi3x þCu2þx .

0.15 0.1 0.05

Bi1.7Pb0.3Sr2Ca2-xYxCu3Oy

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.4

0.5

0.6

Y Content

16 Bi1.7Pb0.3Sr2Ca2-xYxCu3Oy

14 TEP (300 K) (mV/K)

12 10 8 6 4 2 0 -2 -4

487

0

0.1

0.2

0.3 T Content

Fig. 4. (a) Hole-carrier concentrations/Cu ion versus Y content for Bi (Pb):2223 samples. (K) Calculated from values of Tc in superconducting state; (m) calculated from values of TEP in normal state and (b) TEP at room temperature versus Y content for Bi (Pb):2223 samples.

almost linearly up to x ¼ 0.50. Therefore, the disagreement in the behavior of P between the two ways against Y content could be recorded. The interesting fact here is that the above relation is applied in terms of the values of TEP in the normal state. The results of TEP suggested that P decreases by Y substitution in the Bi (Pb):2223 phase and this might cause suppression in Tc, which is eventually expected to be affected by metal–insulator transition. Since the superconducting properties decrease with increase in Y3+ content, it can be said that the hole concentration is expected to decrease due to substitution by Y3+ at Ca2+, in agreement with the values of P estimated from TEP expression. However, the TEP of the Y-doped Bi: 2212 system has been investigated as a function of temperature and it was found that the substitution of Y3+ for Ca2+ decreased the hole concentration than its optimum value [17,35,36]. Furthermore, Wang et al. [37] have found a shift of TEP toward positive values in M-doped Bi:2223 (M ¼ Sm, Gd) systems . They explained these results in terms of decreasing hole-carrier concentration, in agreement with our TEP results. Based on the above results, we noted that the behavior of P, deduced from TEP, against Y is consistent with parabolic behavior, which has been reported for high Tc systems in the under-doped region, and consequently agree with the general roles of substitution as discussed above. However, the vice versa for the behavior of P deduced from Tc values which is not consistent with parabolic behavior for high Tc systems in the under-doped region, and consequently disagree with the general roles of substitution.

So the change of c-axis in the Y-substituted Bi-2223 system is associated with the excess oxygen arising from the replacement of two CaO by one Y2O3 molecule. It has been reported that excess oxygen is taken up by the BiO double planes, causing a tighter binding and hence reducing the c-axis lattice parameter, and consequently superconductivity is depressed [40,41]. On the other hand, the thermal expansion measurements [42,43] based on the BSCCO systems have revealed that the sample with more excess oxygen may be empty from the double Bi–O layers and therefore more positive charges, in the superconducting state, will be transferred to the Cu–O layer. As a result, the hole-carrier concentrations per Cu ion will be increased in the Cu–O layer, in which superconductivity is localized. However, it can be shown that the Tc depression is due to the Y-doped Bi (Pb):2223 phase by considering the following points: (i) X-ray data clearly show the presence of the Bi (Pb):2223 phase, which is decreased by Y substitution; (ii) decreasing c-lattice parameter by Y3+substitution at Ca2+ at the same 8-fold coordination; (iii) the electrical transport is dominated by the Y-doped Bi (Pb):2223 system; (iv) in the absence of any Y substitution, there is a clear step in resistivity at 108 K; (v) decreasing the hole-carrier concentration per Cu ion in the normal state due to substitution by Y3+ at Ca2+; (vi) increasing the holecarrier concentration per Cu ion in the superconducting state as a result of more positive charges transferred to the Cu–O layer. The consistency of these points gives a fair degree of certainty to the suggestion of Y substitution in the Bi (Pb):2223 system.

4. Conclusion The impact of Y substitution on superconductivity in Bi1.7Pb0.3Sr2Ca2xYxCu3Oy samples is investigated. We have shown that Y addition decreased the volume fraction of the 2223 high Tc phase. Further, the critical temperatures are decreased by increasing the Y content up to 0.50. Moreover, the critical concentration of Y to completely suppress superconductivity in Y-doped Bi (Pb):2223 samples is higher than that reported for the other rare-earth elements. The results indicate that the carrier concentration per Cu ion, which is decreased/increased by Y doping in the normal/superconducting state, has a direct connection with the suppression of superconductivity in the considered samples. We suggest that Y, compared with other rare-earth elements, has a higher solubility in the Bi (Pb):2223 system and is less detrimental to the superconductivity.

Acknowledgments The author would like to thank the Physical Department, Faculty of Science, King Faisal University for providing us the experimental facilities required for the present investigation. Also, the author thanks Dr. Sayed Khalil, Physical Department, Sohag University, for his cooperation through TEP measurements. References [1] J.G. Bendnorz, K.A. Muller, Z. Phys. B 64 (1887) 189; H. Maeda, Y. Tanaka, M. Fukutomi, Asano, Jpn. J. Appl. Phys. 27 (1988) L209.

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